Many physiologically active agents are best applied topically to obtain desirable results. Topical application, as contrasted to systemic application, largely avoids side effects of the agents and permits high local concentrations of the agents.
The greatest problem in applying physiologically active agents topically is that the skin is such an effective barrier to penetration. The epidermis of the skin has an exterior layer of dead cells called the stratum corneum which is tightly compacted and oily and which provides an effective barrier against gaseous, solid or liquid chemical agents, whether used alone or in water or oil solutions. If a physiologically active agent penetrates the stratum corneum, it can readily pass through the basal layer of the epidermis and into the dermis.
Although the effectiveness of the stratum corneum as a barrier provides great protection, it also frustrates efforts to apply beneficial agents directly to local areas of the body. The inability of physiologically active agents to penentrate the stratum corneum prevents their effective use to treat such conditions as inflamation, acne, psoriasis, herpes simplex, eczema, infections due to fungus, virus or other microorganisms, or other disorders or conditions of the skin or mucuous membranes, or of conditions beneath the exterior surface of the skin or mucous membranes. The stratum corneum also prevents the skin from absorbing and retaining cosmetic-type materials such as sunscreens, perfumes, mosquito repellants and the like.
Physiologically active agents may be applied to locally affected parts of the body through the vehicle system described herein. Vehicles such as USP cold cream, ethanol and various ointments, oils, solvents, and emulsions have been used heretofore to apply physiologically active ingredients locally. Most such vehicles are not effective to carry significant amounts of physiologically active agents through the skin. One such vehicle is dimethyl sulfoxide, which is described in U.S. Pat. No. 3,551,554. In this description, the term "animal" includes human beings as well as other forms of animal life, and especially domesticated animals and pets.